1. Field of the Invention
The present invention relates to a method for producing toner particles that are used for electrostatic image development in image-forming methods such as electrophotography, electrostatic printing, and magnetic recording. More particularly, the present invention relates to a toner particle production method that, during toner particle production by a wet method, prevents scale from attaching to, for example, the inner walls of the vessel used in the polymerization step and the inner walls of the vessel used in the solvent removal step.
2. Description of the Related Art
Within the sphere of toner particle production methods, for example, the suspension polymerization method and emulsion polymerization and aggregation method, which use a polymerizable monomer and so forth, and the dissolution suspension method, in which a binder resin and so forth is granulated in a solvent, have been actively introduced in recent years for wet-method toners.
For example, in the suspension polymerization method, a polymerizable monomer and a colorant and optionally a release agent, polymerization initiator, crosslinking agent, charge control agent, and other additives are uniformly dissolved or dispersed to prepare a polymerizable monomer composition. This is dispersed, using a suitable stirrer, in an aqueous medium that contains a dispersion stabilizer and the polymerizable monomer is then polymerized to obtain a suspension of toner particles having a desired particle diameter.
In toner production by this suspension polymerization method, the polymerization step is ordinarily carried out using a polymerization vessel that is provided with a stirring means and a heating/cooling means. Scale deposits are formed when during polymerization the polymer composition sticks to, e.g., the inner walls of the vessel, the surfaces of the stirring means, and the baffles, and polymerization occurs in these locations. These scale deposits remain within the polymerization vessel even after the completion of the polymerization step. When these deposits are left in place, they increase in amount in correspondence to the number of consecutive polymerization step batches and lower the heat transfer performance of the polymerization vessel and exercise a negative effect on the stability of the polymerization reaction. In addition, the time required to raise the polymerization temperature and the time required for cooling after the completion of polymerization are increased as the scale deposits grow, resulting in a substantial decline in the productivity. The jacket area ratio with respect to the vessel volume declines in association with an increase in the scale, and as a result the influence of the decline in heat transfer performance also increases as the scale increases. Due to this, the problems of (1) a reduced stability for the polymerization reaction and (2) the increase in the time required for temperature ramp up and cooling are made worse as the scale increases and as a consequence are critical issues with respect to process scale up. Moreover, when the scale deposits exfoliates and fall off and become admixed into the toner product, they are observed as coarse, amorphous particles. When these coarse, amorphous particles take on a large proportion in the toner, this has a negative effect on toner properties, e.g., the triboelectric charging behavior, and on the developing characteristics when image evaluation is performed, causing a decline in product properties, e.g., the appearance of image density variations, white streaks, and fogging is seen, and thus being undesirable.
When these scale deposits exfoliate/fall off after having grown to sufficient size, they cause clogging and sticking in the conduits and valves connected to the polymerization vessel. This then necessitates frequent removal of the deposits, causing a lowering of the availability factor for the production apparatus.
In the dissolution suspension method, which is a different production method from the suspension polymerization method described in the preceding, a toner particle composition is obtained by dispersing or dissolving a toner particle composition, e.g., of a binder resin and colorant and optionally a release agent and other additives, in a volatile solvent, for example, a low-boiling organic solvent. The toner particle composition is then granulated in a dispersing agent-containing aqueous medium and converted into liquid droplets, followed by removal of the volatile solvent. Just as in the polymerization step in the suspension polymerization method described in the preceding, the growth of scale deposits of the toner particle composition occurs on the vessel inner wall in the solvent removal step in the dissolution suspension method, which causes a deterioration in the thermal conduction performance of the vessel and a substantial reduction in the productivity.
With regard to methods for preventing this scale attachment, for example, a method is proposed in Japanese Patent Application Laid-open No. H5-287564 in which an anti-scaling coating is formed on the vessel inner wall by forming an inorganic dispersed powder layer on a layer of an adhesive inorganic compound provided by mixing colloidal silica and an alkyl silicate.
A method is disclosed in Japanese Patent Application Laid-open No. 2006-160960 that prevents the attachment of a polymer scale through the application to the vessel inner wall of an anti-scaling agent that contains a vinylphenol-type polymer and the condensation reaction product of an aldehyde compound and a hydroxynaphthalene-type compound.
With regard to methods of preventing scaling in the gas phase region and gas/liquid interface within a vessel, a method is introduced in Japanese Patent Application Laid-open No. H10-153878 in which water or a dispersion stabilizer-containing aqueous dispersion medium is sprayed during polymerization on the inner wall in the gas phase region in the polymerization vessel.
An anti-scaling method is disclosed in Japanese Patent Application Laid-open No. 2003-287928 in which a dispersion stabilizer-containing aqueous dispersion medium is sprayed on the vessel inner walls at the same time that a preparation is introduced into the vessel, this preparation being obtained by mixing a separately prepared dispersion stabilizer-containing aqueous dispersion medium with a dispersion of the polymerizable monomer composition.
The focus for inhibiting attachment in the means disclosed in Japanese Patent Application Laid-open Nos. H5-287564, 2006-160960, H10-153878, and 2003-287928 rests mainly on increasing the repulsion between the dispersed droplets of the polymerizable monomer and an inorganic dispersed powder or an anti-scaling agent. However, the attachment-inhibiting effect provided by these methods is not adequate and a satisfactory anti-scaling effect is not obtained. In investigations carried out by the applicant, a satisfactory anti-scaling effect was not obtained using these methods, and in particular during continuous production the attachment-inhibiting effect is maintained only for several batches.
Japanese Patent Application Laid-open Nos. 2012-93555 and 2012-93658, on the other hand, introduce several methods for efficiently removing scale deposits once they have been produced. For example, methods in which a removal agent, e.g., an organic solvent or aqueous base solution, is sprayed into the polymerization vessel or is filled into the polymerization vessel and heating and stirring are used in combination therewith are convenient and common. However, with all of these methods, the focus is how well the scale deposits are dissolved or swollen in a solution, and even with heating an extended period of time is required to a certain degree in order to completely remove the deposits.